1. Field of the Invention
This invention relates to a cryogenic apparatus such as a cryostat for a liquid helium immersed type superconducting magnet used in a magnetic resonance imaging apparatus.
2. Description of the Prior Art
Recently, superconductive magnetic resonance imaging apparatuses have been in practical use. In such apparatuses, a superconductive magnet is cooled by liquid helium. To reduce the evaporation of the liquid helium, it is suggested to cool a radiation shield, which encloses a refrigerant vessel containing a superconducting magnet and refrigerant, by a refrigerator.
In this case, however, the following problems arise. The temperature of the refrigerator at the cooling stage is so cool that air is frozen. If an impurity enters an operating fluid path at the time of routine replacement of a seal member which is usually provided in the refrigerator, it is liable to be frozen in a low temperature section of the path, thus giving rise to various problems. If the temperature of the refrigerator rises in order to melt the frozen impurity, the temperature of the super-conducting magnet and radiation shield also rises. Particularly, in case where the impurity is moisture, the temperature of the refrigerator has to be risen up to normal temperature to melt the frozen impurity. This rise of temperature causes temperature of the superconducting magnet and radiation shield to rise up to the neighborhood of normal temperature. To resume operation of the imaging apparatus, it is necessary to cool again the superconducting magnet having risen in temperature. The time required for re-cooling and consumption of the refrigerant constitute problems in case of providing a practical construction where the radiation shield is cooled by the refrigerator.